In many industrial and medical applications, virtually complete removal of contaminants from the surfaces of articles or devices is essential. Particularly acute are the cleaning requirements for medical devices used in a wide array of medical procedures, and for electrical or electronic assemblies in which metal/plastic combinations pose numerous cleaning problems and cleaning agents must penetrate the microscopic crevices of electronic chips. It is with particular reference to such endeavors that the ensuing description will be made; however, it will be appreciated by those skilled in the art that similar problems of contaminant removal can arise in myriad other applications.
The use of chlorofluorocarbons (CFC's) as solvents, cleaning agents or degreasing agents is common in industry for the removal of oil, grease, and related contaminants from articles and materials. These solvents have been generally favored because they are easily volatilized, are inert to chemical reaction with most materials, are colorless, virtually odorless, nonflammable, noncorrosive, highly stable and have low toxicity compared to alternatives.
Despite the many advantages of chlorofluorocarbons as solvents, they are not well suited for removing some types of contaminants, such as resins and polar compounds. Other solvents are generally required for these substances. The lower aliphatic alcohols, such as methanol, ethanol and isopropanol, have long been used for cleaning and removing resins and polar substances. Their excellent cleaning properties have even led to military specifications whereby alcohols are used to assess the cleaning properties of other liquids. Use of alcohols, however, poses high flammability and explosion risk. Specially built and maintained facilities are required to reduce the chance of fire or explosion. Flame, spark and explosion proofing (for example, the use of bronze motors) is extremely costly, making it difficult to utilize alcohols by inexpensive means.
To overcome the risks and expense associated with use of alcohols, yet provide a system for removal of both oil-based and polar contaminants, special non-flammable solvent blends have been formulated. For example, such blends are commercially available as TP-35 (35% isopropanol in trichlorotrifluoroethanes) and Freon TP (6-8% isopropanol in trichlorotrifluoroethanes) manufactured by E. I. Du Pont de Nemours & Company, Wilmington, Del; and are described in U.S. Pat. No. 3,789,004 issued Jan. 29, 1974 to McMillan et al. (94% 1,1,2-trichloro-1,2,2-trifluoroethane, 3% ethanol and 3% acetonitrile); and U.S. Pat. No. 4,745,690 issued May 24, 1988 to Koop et al. (fluorocarbon, such as 1,1,2-trichloro-1,2,2-trifluoroethane, and alcohols, such as isopropanol, methanol, ethanol and butanol). These formulations are reportedly nonflammable and generally stable liquids, but are limited in the percentage of total alcohol in the solution. If a higher percentage of alcohol is required to remove a specific contaminant, such solvent blends are not efficient cleaners.
Other cleaning systems have been described which either improve or extend the use of chlorofluorocarbons to remove certain types of contaminants, or reduce the flammability risk of using alcohols. A multisolvent-compartment system in which one compartment contains a chlorofluorocarbon with a surfactant is one such approach. In this system, articles to be cleaned are first subjected to an aqueous cleaning and are then immersed in chlorofluorocarbon compartments, one of which contains chlorofluorocarbon with the surfactant to remove water. It is reported that the addition of surfactant to the chlorofluorocarbon ensures removal of traces of water by solubilization. (See, Banks, Organofluorine Chemicals and Their Industrial Applications, Ellis Horwood Ltd., West Sussex, England, 1979, p. 70, citing British Standard Specification BS4849).
An approach which limits the flammability risk of alcohols as cleaning agents is described in "Alcohol Cleaning under a Nonflammable Perfluorocarbon Vapour Blanket" by Slinn and Baxter, Proceedings of the Technical Program, Nepcon West '90, Feb. 26-Mar. 1, 1990. This system basically consists of floating a less dense alcohol layer, such as isopropanol, on a more dense boiling perfluorocarbon layer. The vapor above the liquids is a mixture of the perfluorocarbon and isopropanol and is nonflammable. Two sets of cooling coils are utilized in the vapor space above the liquids, one to recover the alcohol and a second for the perfluorocarbon. Although this system has the advantage of using a perfluorocarbon which has an apparently comparatively negligible ozone-depletion potential, it has drawbacks with regard to the cleaning operation. For example, the temperature of the liquid alcohol is limited by the boiling temperature of the perfluorocarbon, which will be well below the boiling temperature of the alcohol, thus eliminating cleaning situations which may require high temperatures.
Despite attempts to provide simple, nonflammable, nonexplosive, low toxicity, cost efficient methods of removing a full spectrum of contaminants from grease- and oil-based to polar, the art has not responded with the introduction of a system having features that adequately address these considerations.